Electrostatic printing ink and process for preparing same



Oct. 18, 1966 1 R. s. HOWELL 3,280,036

ELECTROSTATIC PRINTING INK AND PROCESS FOR PREPARING SAME Original Filed July 31, 1959 2 Sheets-Sheet l 0 ATTORNEY INVENTOR. RICHARD S. HOWELL Oct. 18, 1966 R. s. HOWELL 3,230,036

ELECTROSTATIC PRINTING INK AND PROCESS FOR PREPARING SAME Original Filed July 31, 1959 2 Sheets-Sheet 2 IBX 25 ,/I4

DATA SOURCE DECODER BI CONTROL SIGNAL GENERATOR PRINT PULSE GENERATOR INITIATING PULSE GENERATOR IN V EN TOR.

6 RICHARD S. HOWELL ATTORNEY United States Patent 3,280,036 ELECTROSTATIC PRINTING INK AND PROCESS FOR PREPARING SAME Richard S. Howell, King of Prussia, Pa., assignor to Burroughs Corporation, Detroit, Mich, a corporation of Michigan Original application July 31, 1959, Ser. No. 830,772, now Patent No. 3,094,429, dated June 18, 1963. Divided and this application Sept. 21, 1961, Ser. No. 139,784 21 Claims. (Cl. 252-621) The present application is a division of my co-pending patent application Serial No. 830,772, for Colored Ink for Electrostatic Printing, filed July 31, 1959, and assigned to the Burroughs Corporation, the assignee of the present invention, and now US. Patent No. 3,094,429.

This invention pertains to developing pigments or inks for rendering visible electrostatic charge patterns and to the manner of their preparation and use.

In application for United States patent Serial No. 784,- 300, entitled Electrostatic Recording, filed December 31, 1958, by David W. Seymour, and assigned to the assignee of this application, there is taught the use of a particulate ink of moderate density, having an electrically conductive outer surface, and contrasting in some color with that of the surface of the electrostatic recording medium used. However, the embodiments there described were limited in the range of possible colors, since the coloring means employed had to possess certain physical and electrical characteristics which were not generally found in conventional dyes.

I have found that, by modifying and improving the process taught by Seymour, I can use conventional dyes to color ink particles which have been found, by test in conventional electrostatic printing equipment, to function as completely satisfactory pigments or inks for rendering visible patterns of electric charges deposited on dielectric record media. Furthermore, by providing inks of distinctly different ranges of particle size and of diiferent colors, I can cause the coarser ink to adhere to the region of greater charge density, thus effecting electrostatic printing or recording in different colors.

I have found further that an ink comprising porous, inorganic, substantially spherical par-ticles when coated and partially saturated with a mixture comprising a dye and an electrically conductive material presents an ideal ink for rendering visible patterns of electric charges deposited on dielectric record media.

Thus, an object of my invention is to provide an improved ink for developing electrostatic charge patterns.

Another object of my invention is to provide a fluid collection of colored solid ink particles sufliciently conductive to develop electrostatic charge patterns on dielectric media whichink is non-friable to permit fixing pressure, wherein the conductive surface material is uniformly disposed on the base material and firmly adheres thereto and wherein the particles may be made uniform and in a predetermined range.

Another important object of my invention is to provide an ink or pigment for developing or rendering visible electrostatic charge patterns in any one of a large number of colors.

Another important object of my invention is to provide ink suitable for development of electrostatic charge patterns of varying density in colors dependent upon the density of the electrical charge developed.

Other objects and benefits of my invention will appear in the following description and specification.

For the better comprehension of my invention, I provide figures of drawing as follows:

FIGURE 1 represents enlarged views of particles of clay cracking catalyst covered with colored coating, according to my invention;

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FIGURE 2 represents a dielectric medium bearing a charge pattern of varying densities, invisible because undeveloped;

FIGURE 3 represents the dielectric medium of FIG- URE 2 undergoing selective development by a process and apparatus utilizing the ink of my invention;

FIGURE 4 represents the dielectric medium of FIG- URE 2 undergoing development by an alternate process and apparatu using the ink of my invention;

FIGURE 5 represents the dielectric medium of FIG- URE 2 after development according to the process of FIGURE 3 or of FIGURE 4, showing a reproduction in two colors of the charge patterns represented in FIG- URE 2;

FIGURE 6 represents a means for applying charge patterns of varying density to a dielectric recording medium; and

FIGURE 7 represents a detail of the means for applying charge patterns of varying density to a dielectric recording medium.

As is explained in detail in the application of Seymour, previously referred to, the base material for a particulate ink should preferably have a specific gravity of the order of three or four referred to water, and be sufiiciently nonfriable to sustain fixing pressure. For the purposes of optimum use in the electrostatic process of the ink of my invention it is desirable that the base material be either highly absorbent or porous or both, in order to produce firm adherence of the electrically conductive dye mixture to the base. While some glasses are hygroscopic, usual commercial glasses tend to be resistive to water and weak chemicals, so that the adherence of a conductive dye mixture to a good grade of glass is likely to be poor. Porous nonvitreous ceramics, on the other hand, are good absorbers of the dye mixture.

A suitable material for my improved ink is the fluid solid particles of porous clay known in the petroleum refining industry as fluid clay cracking catalyst. These comprise a spray-dried mixture of finely divided aluminasilica gel catalyst. The Davison Chemical Company, of Baltimore 3, Maryland, produces such material commercially; their type F2 catalyst includes approximately 4 percent by weight of particles less than 20 microns in average diameter, approximately 8 percent by weight of particles of average diameter greater than 74 microns, and approximately 88 percent by weight of particles having diameters averaging between 20 and microns. For ink to be used in developing charge patterns of approximately uniform density, a range of sizes between twenty and eighty microns has been found desirable. The clay catalyst particles greatly in excess of eighty microns may be removed by standard screening procedures, and those considerably under twenty microns in diameter may be removed by air elutriation, which is also a standard technique Widely employed in the processing of finely divided materials such as ceramic raw materials, and Portland cement. The coloring materials I employ are standard water-suspendible organic dyes, such as the Carmoisine, Acid Green (see Fisher Chemical Index, p. 123, copyright 1959, The Fisher Scientific Co.) and Alizarine Light Blue, sold commercially by Sandoz, Incorporated, of 63 Van Dam. Street, New York, N.Y. In order to insure uniform deposition of the dyes upon the clay surface, I find it expedient to add to the water in which the dye is suspended a relatively viscous material such as glycerine; and to achieve a modicum of electrical conductivity, I add a small amount of preferably hygroscopic salt, such as lithium chloride, to the water. Since weak acids or bases would not add much to conductivity and strong ones are corrosive, salts are preferable. Any conductivity producing water-soluble material may be used.

The lithium chloride may be dissolved in the water, the

glycerin may then be stirred into the solution, and the dye may be added, in that sequence. The water-glycerinelithium chloride-dye solution is then brought approximately to the boiling point-to a temperature of about two hundred degrees Fahrenheit, for exampleand the clay catalyst particles are then added to the hot solution and stirred into the liquid until the mixture behaves like a somewhat viscous paste, such as toothpaste. I have found the following proportions satisfactory for this purpose, although it is obvious that variations are possible:

Water milliliters 900 Glycerin do 50 Lithium chloride grams 1 1 Dye do Clay catalyst to make thin paste (approx. 1000 grams).

The glycerin may be replaced with diethylene glycol, and the lithium chloride may be replaced with zinc chloride or calcium chloride. Other polyhydric alcohols or other polar water-soluble compounds of suitable fluidity and boiling point may replace glycerin.

The mixture is spread out on a suitable surface and dried in an air drying oven. The resulting product is a fluid collection of solid particles, colored by the dye, and sufficiently conductive to be used to develop electric charge patterns on dielectric media having conductive backing layers, according to the teaching of co-pending application for United State patent Serial Number 714,767, filed February 12, 1958, by Robert E. Benn and Herman Epstein, entitled Electrographic Recording Process, which is assigned to the assignee of this application, and now abandoned. It is explained in detail in the Benn-Epstein application how a great benefit may be achieved by providing a conductive backing layer on a dielectric record medium, so that the effective surface capacitance of the dielectric medium will be increased; and how the use of a conductive ink for developing a charge pattern on such a record medium permits the flow of bound charges from the back conductive layer into the conductive ink directly opposed to the stored charge pattern.

The arbitrary method employed for measuring the bulk resistance of electrographic inks, as described by Seymour in his aforesaid co-pending application for patent, consists in placing in an insulating circular cylinder one inch in diameter with a metal terminal plug in the bottom sufficient ink to fill the cylinder to a depth of one inch when a total force of one hundred grams is applied to the top of the mass of ink. With this total force of one hundred grams compressing the ink, the electrical resistance between metal plug contacts at the bottom and the top of the ink mass is measured. This method is purely arbitrary and has no physical significance other than empirical. Seymour reported that values from 1,000 to 10,000 ohms were satisfactory. I find that ink made according to my present invention functions satisfactorily even when measurements made as described indicate it to have a bulk resistance of approximately a megohm. However, the currents flowing in the electrostatic developing process are so extremely minute by comparison with the currents passed through a bulk of ink during measurement of the electrical resistance of such a bulk that it is possible that the correlation between measured bulk resistance and effective resistance during actual development is very poor, and that the resistivities actually manifested in use by the carbon-coated ink of Seymour and the dyed ink of my own invention are much more nearly alike than bulk measurements indicate.

Since ink made according to my invention may be produced in any color for which a suitable dye is available, it becomes possible to consider means for printing electrostatically in different colors. It is true that the United States patent to Selenyi, 2,143,214, entitled Production of Images, teaches the possibility of producing simultaneous deposits of minium and sulphur on charge patterns sense comprising two different polarities of charge; but this is a somewhat fortuitous result of the electrical properties of two materials which happen to have different colors, and does not open the way to printing in two arbitrarily chosen colors and in using the same material for the cores of the particles. It is, of course, possible to print electrostatically in any number of different colors by applying first the pattern to be printed in a given color, developing that with ink of the desired color and fixing the ink, and then applying, developing and fixing a pattern developed in ink of a second color, and so on. However, the advantages of printing in two or more different colors, such as black and red, are sufficient to justify slight additional complexity in the present standard single-color electrostatic printing process to achieve at least twocolor printing.

1 have found that it is possible to prepare ink in different particle sizes, and to achieve selective attachment of these different particles to regions of different charge density. For example, ink having a particle size ranging from forty to fifty microns, and ink having a particle size ranging from one hundred to one hundred ten microns, will behave differently, when applied to patterns of electrostatic charges of differing densities. It is possible to apply charges differing in density either by altering the voltage employed to deposit the charges on the record medium, or by employing a fixed voltage and limiting the quantity of charge available for deposition in a given time. The latter procedure is preferable, since the maximum possible density obtainable (which necessarily determines the minimum spot size producible for a given magnitude of charge) is limited by the voltage used to store the charge; and generally the smallest possible spot size is desirable.

The total charge on a sphere tends, other things being equal (which practically never occurs), to be proportional to the surface area of the sphere, and the mass of the sphere for a given density is proportional to the cube of its radius, or the 3/2 power of its area. Thus the larger the diameter of the sphere, the stronger the field required to attach it to the surface of the record medium, in general. Thus the larger diameter link particles will be retained only by the higher density charges, while the smaller diameter ink particles will be retained by both the high and the low density charges.

FIGURE 3 represents a device for producing two-color electrostatic printing by the methods here outlined. Recording medium 14, having a highly insulating dielectric surface with an electrically conductive backing is fed from a storage reel 16 through a printing station 15, represented in more detail in FIGURES 6 and 7, and around an idler 25 into a first developing station comprising a container 17 partly filled with a mass of ink 18. The ink 18 is of large particle size, e.g. from ninety to one hundred twenty micron-s average diameter, and of a first color coating. The heavily charged areas of medium 14 will be covered with adherent particles of ink 18 during passage of medium 14 through the ink 18. Upon rising clear of the mass of ink 18, the medium passes between baffles 19 which prevent any accidental purely mechanical adhesions of ink from passing up with the medium 14, and medium 14 is also engaged and shaken by a polygonal rotating beater 20, driven by means not here shown, which shakes loose any particles or aggregations of ink 18 accidentally adhering to any part of medium 14 where there is not in fact a charge of high density. The point of entrance of the medium 14 through the bottom of vessel 17 may be kept tight by the use of wipers of felt or similar material which will permit the passage of the medium upward without allowing the downward flow of the ink 18. Similarly the point of entrance of medium 14 through the bottom of vessel 21 may be kept tight by the user of wipers, but these must not fit so tightly that they will wipe off the particles of ink 18.

The medium 14, having been developed in a first color by ink 18, then enters a vessel 21, containing a quantity of ink 22, ranging in average particle diameter from twenty to sixty microns, and dyed a second color. The ink 22 adheres to the charged area of the dielectric surface of medium 14 not yet covered with ink 18; that is, primarily to the areas of low-density charges. After passing between bafiles 23 similar in design and purpose to baffies 19, and against a beater 24 similar in design and purpose to beater 20, the medium passes around an idler 25 and between fixing pressure rolls 26 which compress the particles of ink against the surface of medium 14 so as to embed the ink in the medium and thus fix it.

FIGURE 4 represents an arrangement very similar to that of FIGURE 3, except that the single developing chamber contains an ink 27, composed of a mixture of ink-s 18 and 22. I have found that, when a mixture of large and small particle sizes is used in developing ink, while the small particle sizes alone cling to the areas of low charge density, the high-density areas attract both large and small sized particles. However, the larger particles tend to conceal the smaller ones, so that the predominant color impression produced is that the areas of high charge density are covered with ink of one color and the areas of low charge density are covered with ink of the other color. Thus passage of the medium 14, hearing a record in two different charge densities applied by recording station 15, through the ink mixture 27 results in the application of inks 18 and 22 selectively to the regions of differing charge density, thus producing a two-color development of the electrostatic charge pattern on the medium. The baffles 19 and heater 20 serve the same function as in FIGURE 3; and, after passage over an idler 25 the medium will undergo fixing of its two-color developed pattern by the pressure of the opposed rolls 26.

FIGURE 5 represents, developed in two colors with inks 18 and 22, the same charge pattern on medium 14 represented undeveloped in FIGURE 2. This substantial result may be obtained either by the method of FIGURE 3 or by that represented by FIGURE 4.

It is, of course, possible to employ the basic principle of charge-density alteration and difierences in ink particle size to achieve selective deposition of two inks differing in properties other than color-for example, it is taught by Seymour in his aforesaid patent application that the selective surface Wetting characteristics of inks may be used to produce, by electrostatic printing, masters for reproduction by lithographic means.

FIGURE 6 represents schematically an arrangement for producing charge deposits of different densities. FIG- URE 7 represents a detailed view of the printing head 39 represented as employed in FIGURE 6. This type of printing head is discussed in detail in application for United States patent Serial Number 734,253, filed May 9, 1958, by Robert E. Benn, Richard S. Howell and Richard S. Sakurai, entitled Electrographic Recording Appa ratus, and assigned to the assignee of the present invention, and now US. Patent No. 3,068,479. Print head 39 has a body 40 of insulating material, which carries at least partially embedded therein an extended print electrode 41 and a plural number of associated initiating pin electrodes 42 extending alongside of the print electrode. It is understood that the print head may contain more than one print electrode and associated initiating electrodes. The basic mode of operation of the head employs a lowi-mpedance voltage pulse source, known as a print pulse generator, connected to print electrode 41. High-impedance voltage pulse sources, or current pulse sources, are connected individually to initiating electrodes 42. A grounded electrode, such as anvil 38 of FIGURE 6, is opposed to the electrodes of head 39, and the path of movement of the dielectric record medium 14 is between the head and the anvil.

The amplitude of the print voltage pulse applied between print electrode 41 and anvil 38 is insufficient to cause an electrical discharge to occur in the ambient atmosphere around the electrodes. However, it is of sufficient magnitude so that if electric charges are provided in the ambient atmosphere, those of appropriate polarity Will be driven toward the anvil 38, but will be stopped by deposition on medium 14. Initiating electrodes 42, five being shown, are employed to provide electric charges at selected points along print electrode 41. The highimpedance initiating pulse voltage sources connected to electrodes 42 produce pulses of polarity opposite to the printing voltage pulse applied to print electrode 41; and the initiating pulse sources have sufficiently high impedance so that they behave at least approximately like current sources. Thus an initiating pulse applied to a given pin electrode 42 will cause a breakdown of the ambient atmosphere between print electrode 41 and the initiating electrode 42, with a discharge at a given current, but without particularly affecting the potential of print electrode 41, which is fed from a low-impedance source. The simultaneous application of a print voltage pulse to the print electrode 41 and of an initiating current pulse to an initiating electrode 42 wil cause deposition of an electrostatic charge on the dielectric surface of medium 14 approximately opposite the space between the two electrodes 41 and 42. The density of such a charge may be modified by one (or both) of two means. If the amplitude of the printing voltage pulse is reduced by a considerable factor (e.g. reduction from 1200 volts to 400 volts has been found satisfactory), the density of deposited charge will vary sufficiently to produce selective adherence of inks of 40 to 50 microns average diameter, and of inks of to microns average diameter. Alternatively, the total charge available from each initiating discharge may be varied by altering the current in or duration of each initiating pulse, keeping the printing voltage pulse amplitude constant. This latter procedure has the advantage that the definition of the deposited charge spot is less unfavorably affected by variations in available charge than by variations in printing voltage. Combination of both methods is, of course, also possible.

FIGURE 6 represents means for practicing either of these methods, or both. A reel 37 carries a supply of dielectric tape medium 14 which is fed by means not shown between grounded anvil 38 and electrostatic printing head 39, detailed in FIGURE 7 as above described. Printing electrode 41 is connected by conductor 33 to print pulse generator 32, which is grounded and is connected by conduct-or 31 to decoder and control signal genera-tor 30. Initiating electrodes 42 are individually connected by conductors 36 to separate selector and initiating pulse generators 35, which are grounded and are also connected by conductors 34 to decoder and control signal generator 30. The use of simple rectangles to represent electrical elements is permissible because the state of the electrical art is such that, given the inventive concept calling for particular voltages of pulses produced at particular times through specified impedances, the implementation of the inventive concept may be achieved in a number of ways according to well known art. The application for United States patent of Benn, Howell and Sakurai to which reference was previously made herein gives complete information for the construction of print and selecting pulse generators and the associated equipment for their use. However, reference to various volumes of the Radiation Laboratory Series, particularly volume 5, Pulse Generators, by Glasoe and Lebacqz, dated 1949, and volume 19, Waveforms, by Chance, Hughes, MaoNichol, Sayre and Williams, dated 1949, all published by the McGraw-Hill Book Company, Incorporated, of New York city, N. Y., is made here to establish the vast extent of the known art.

In the first contemplated mode of operation of the device represented in FIGURE 6, data source 28 transunits via channel 29 (which may be a single conductor as here implied, or any one of many equivalents) the data to be printed, which are received by decoder and control signal generator 3%. The decoder and control signal generator is specified as containing timing means for determining the appropriate time for transmission of the various control signals of which it is capable. It is also required to be capable of performing the function of decoding incoming signals from data source 28 to determine which characters should be printed in response to these signals, and of exciting via lines 34 the appropriate ones of initiating pulse generators 35 to cause via lines 36 discharges at the proper ones of initiating electrodes 42 to produce charge deposition at the appropriate place and at the correct time to produce charge patterns as required for a given symbol. During the time of the initiating discharges the control signal generator also must excite the print pulse generator 32 by channel 31 and cause it to apply via channel 33 a printing pulse to print electrode 41.

The sequence of operation outlined thus far would result in conventional electrostatic printing, at approxi mately uniform charge density. However, let the specification of the print pulse generator the amended to require that it be capable of applying, upon appropriate signal, either of two different pulse voltages to the print electrode 41, and that the control signal generator 30 be capable of providing such selective signals via channel 31. Then when a high-density pattern is to :be printed, the print pulse generator 32 will be controlled to produce a highvoltage printing pulse; and when a low-density pattern is to be printed, the control signal generator 30 will control the print pulse generator 32 to cause it to produce a lower voltage printing pulse of, e.g. one-half the higher value. As an alternative, let the specifications of the initiating pulse generators 35 be amended to provide that they be capable of providing pulses of two different values of current. Since the approximation to constant current pulses is achieved by causing the initiating pulse generators to produce a pulse of given voltage amplitude through a high impedance, the same design feature which can permit a print pulse generator to produce pulses of either of two voltage amplitudes can permit an initiating pulse generator to produce pulses of either of two current amplitudes. One rather primitive but straightforward way of producing on a given terminal pulses of two amplitudes is to employ two separate pulsing circuits each adapted to give out a particular voltage, and to tie the two circuits to the given terminal through series output diodes, commonly known in such use as or" gates or butters. (This is a part of standard electronic design and involves only known art.) Furthermore, let the specification of the decoder and control signal generator 30 be amended to provide that it be capable of designating in its function of controlling the initiating pulse generators 35 not only which generators shall fire, but whether they shall produce high or low current pulses. Then, with the print voltage pulse at a fixed amplitude, a low-current initiating discharge will make available for deposition only a limited quantity of charge, producing a low-density charge pattern adapted to cause the adherence of only small-diameter ink particles; and a highcurrent initiating pulse will make available a larger quantity of charge and produce a high-density charge pattern adapted to cause the adherence of ink particles of larger diameters.

A third alternative for depositing charge patterns of diifering densities is to cause the printing voltage pulse and the initiating current pulse to be decreased simultaneously in order to produce low-density charge deposition. However, the ideal definition will be most nearly approached by the use of the highest feasible printing potential and control of charge density by limitation of available charge, according to well known physical principles.

It is thus evident that a wide variety of colors of particulate ink, of different average diameters of particles, may be produced at will for use as developers of electrostatic charge paterns; and that, by the use of electric patterns of different charge densities, it is possible to selectively develop such patterns with two or more different kinds of ink.

The invent-ion provides further an improved ink suitable for developing electrostatic charge patterns on dielectric media which ink comprises a fluid collection of uniformly coated, conductively dyed, solid, non-friable, particles of predetermined suitable range and uniformity of size and wherein the conductive dye adheres well to the base particles.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced other than as specifically described and illustrated.

What is claimed is:

1. An electrostatic charge pattern developing ink comprising pellets of alumina-silica gel catalyst between 20 and 120 microns in diameter, coated and partially saturated with a mixture of a water-suspendible, organic dye, a polar, water-soluble, organic, viscous material for facilitating the coating of said pellets by said dye and a hygroscopic salt capable of ionizing and in suflicient quantity to render said ink conductive.

2. A particulate ink for developing electrostatic charge patterns consisting essentially of substantially spherical pellets of alumina-silica gel catalyst of diameter substantially between 20 and microns, coated with a mixture of a Water-suspendible organic dye, glycerin in sufficient quantity to facilitate even coating of said pellets by said dye and lithium chloride in sufiicient quantity to render said ink conductive.

3. A particulate ink for developing electrostatic charge patterns consisting essentially of substantially spherical pellets of alumina-silica gel catalyst of diameter substantially between 20 and 80 microns, coated with a mixture of a water-suspendible organic dye, diethylene glycol in suflicient quantity to facilitate even coating of said pellets by said dye and lithium chloride in sufficient quantity to render said ink conductive.

4. A particulate ink for developing electrostatic charge patterns consisting essentially of substantially spherical pellets of alumina-silica gel catalyst of diameter substantially between 20 and 80 microns, coated with a mixture of a water-suspendible organic dye, glycerin in suificient quantity to facilitate even coating of said pellets by said dye and zinc chloride in sufiicient quantity to render said ink conductive.

5. A particulate ink for developing electrostatic charge patterns consisting essentially of substantially spherical pellets of alumina-silica gel catalyst of diameter substantially between 20 and 80 microns, coated with a mixture of a water-suspendible organic dye, glycerin in sufficient quantity to facilitate even coating of said pellets by said dye and calcium chloride in sufficient quantity to render said ink conductive.

6. A particulate ink for developing electrostatic charge patterns consisting essentially of substantially spherical pellets of alumina-silica gel catalyst of diameter substantially between 20 and 80 microns, coated with a mixture of a water-suspendible organic dye, diethylene glycol in suflicient quantity to facilitate even coating of said pellets by said dye and zinc chloride in sufiicient quantity to render said ink conductive.

7. A particulate ink for developing electrostatic charge patterns consisting essentially of substantially spherical pellets of alumina-silica gel catalyst of diameter substan tially between 20 and 80 microns, coated with a mixture of a water-suspendible organic dye, diethylene glycol in sufiicient quantity to facilitate even coating of said pellets by said dye and calcium chloride in sufficient quantity to render said ink conductive.

3. A process of preparing an ink for developing electrostatic charge patterns comprising: screening particles of nonvitreous, absorbent, non-friable ceramic in a range between 20 and 120 microns in diameter, heating to about boiling point water containing a water-suspendible organic dye, a water-soluble salt capable of ionizing and in suflicient quantity to render said ink conductive, and a sufficient amount of viscous, polar, water-soluble, organic material to facilitate uniform coating of said particles, mixing said screened particles of nonvitreous ceramics in the Water to a viscous paste consistency, and drying to a relatively conductive fluid collection of solid particles. I

9. The process of claim 8 in which said salt is Zinc chloride.

10. The process of claim 8 in which said salt is calcium chloride.

11. The process of claim 8 in which said viscous material is diethylene glycol.

12. The process of claim 11 in which said salt is zinc chloride.

13. The process of claim 11 in which said salt is calcium chloride.

14. A process of providing a fluid collection of solid dye colored particles, sufliciently conductive to be used to develop electrically charged patterns on dielectric media having conductive backing layers comprising: screening alumina-silica gel catalyst particles in excess of 80 microns and removing particles under 20 microns by air elutriation, dissolving sufficient ionizable, hygroscopic salt into water to provide an electrically conductive solution, mixing into said solution a polyhydric alcohol in sufiicient quantity to facilitate coating of said particles, adding water-suspendible organic dye into said solution, bringing the solution containing the mixture of water, polyhydric alcohol and salt to the boiling point, adding said catalyst particles to the hot solution and stirring the liquid until the mixture behaves like a viscous paste, spreading the viscous paste mixture out on a surface and drying.

15. A process of preparing an ink for developing electrostatic charge patterns comprising: screening particles of a spray-dried mixture of finely divided alumina-silica gel catalyst, heating to about boiling point water containing a water-suspendible organic dye, lithium chloride in sufiicient quantity to render said ink conductive and a suflicient amount of glycerine to facilitate uniform coating of said particles, mixing said screened particles of alumina-silica gel catalyst in the water to a viscous paste consistency, and drying to a relatively conductive fluid collection of said particles.

16. The process of claim 15 wherein the mixture materials are in approximate proportions of 900 millilitres of water, 50 millilitres of glycerine, 11 grams of lithium chloride, grams of dye, and 1000 grams of catalyst.

17. A process for preparing an ink for developing electrostatic charge patterns comprising: screening particles of aluminna-silica gel catalyst in a range between 20 and microns in diameter, heating to about boiling point water containing a water-suspendible organic dye, zinc chloride in sufiicient quantity to render said ink conductive, and a sufficient amount of viscous, polar, watersoluble, organic material to facilitate uniform coating of said particles, mixing said screened particles in the water to a viscous paste consistency, and drying to a relatively conductive fluid collection of solid particles.

18. A process of preparing an ink for developing electrostatic charge patterns comprising: screening particles of aluminna-silica gel catalyst in a range between 20 and 120 microns in diameter, heating to about boiling point water containing a water-suspendible organic dye, calcium chloride in sufficient quantity to render said ink conductive, and a sufficient amount of viscous, polar, watersoluble, organic material to facilitate uniform coating of said particles, mixing said screened particles in the water to a viscous paste consistency, and drying to a relatively conductive fluid collection of solid particles.

19. A process of preparing an ink for developing electrostatic charge patterns comprising: screening particles of alumina-silica gel catalyst in 'a range between 20 and 120 microns in diameter, heating to about boiling point Water containing a water-suspendible organic dye, a water soluble salt capable of ionizing and in sufiicient quantity to render said ink conductive and a sufficient amount of diethylene glycol to facilitate uniform coating of said particles, mixing said screened particles of alumina-silica gel catalyst in the Water to a viscous paste consistency, and drying to a relatively conductive fluid collection of solid particles.

20. The process of claim 19 in which said salt is zinc chloride.

21. The process of claim 19 in which said salt is calcium chloride.

References Cited by the Examiner UNITED STATES PATENTS 1,755,616 4/1930 Vanderbilt 117-100 2,638,416 5/1953 Walkup et a1. 25262.1 2,758,524 4/1956 Sugarman 11717.5 2,758,939 8/1956 Sugarman 252--62.1 2,916,344 12/ 1959 Kukin et al 10622 2,935,481 5/1960 Hochwalt 106-308 3,075,859 1/1963 Relph et al 11717.5

LEON D. ROSDOL, Primary Examiner. JULIUS GREENWALD, Examiner.

S. R. BRESCH, K. VERNON, R. D. LOV-ERING,

Assistant Examiners. 

1. AN ELECTROSTATIC CHARGE PATTERN DEVELOPING INK COMPRISING PELLETS OF ALUMINA-SILICA GEL CATALYST BETWEEN SATUAND 120 MICRONS IN DIAMETER, COATED AND PARTIALLY SATURATED WITH A MIXTURE OF A WATER-SUSPENDIBLE, ORGANIC DYE, A POLAR, WATER-SOLUBLE, ORGANIC, VISCOUS MATERIAL FOR FACILITATING THE COATING OF SAID PELLETS BY SAID DYE AND A HYGROSCOPIC SALT CAPABLE OF IONIZING AND IN SUFFICIENT QUANTITY TO RENDER SAID INK CONDUCTIVE.
 8. A PROCESS OF PREPARING AN INK FOR DEVELOPING ELECTROSTATIC CHARGE PATTERNS COMPRISING: SCREENING PARTICLES OF NONVITREOUS, ABSORBENT, NON-FRIABLE CERAMIC IN A RANGE BETWEEN 20 AND 120 MICRONS IN DIAMETER, HEATING TO ABOUT BOILING POINT WATER CONTAINING A WATER-SUSPENDIBLE ORGANIC DYE, A WATER-SOLUBLE SALT CAPABLE OF IONIZING AND IN SUFFICIENT QUANTITY TO RENDER SAID INK CONDUCTIVE, AND A SUFFICIENT AMOUNT OF VISCOUS, POLAR, WATER-SOLUBLE, ORGANIC MATERIAL TO FACILITATE UNIFORM COATING OF SAID PARTICLES, MIXING SAID SCREENED PARTICLES OF NONVITREOUS CERAMICS IN THE WATER TO A VISCOUS PASTE CONSISTENCY, AND DRYING TO A RELATIVELY CONDUCTIVE FLUID COLLECTION OF SOLID PARTICLES. 